Field of the Invention
This invention relates to a method of decomposing halogenated aliphatic hydrocarbon compounds and also to an apparatus to be used for such a method.
Related Background Art
With developing industries, a large amount of organic compounds, such as halogenated aliphatic hydrocarbon compounds, have been used, and the disposal of such compounds has become a serious problem. In particular, spent halogenated aliphatic hydrocarbon compounds are causing great concern over environmental pollution. Extensive efforts have been made to solve this problem.
For example, chlorinated aliphatic hydrocarbon, compounds such as trichloroethylene (TCE) and tetrachloroethylene (PCE), belong to halogenated aliphatic hydrocarbons, which have been abundantly used in various industries as cleansing solvents for metal parts, semiconductor parts and textiles and also as reaction solvents.
However, as their toxicity, such as mutagenicity and carcinogenicity, has been pointed out, complete prohibition of their use and treatment to make the spent solvents harmless have been required. Additionally, the compounds already released into the natural environment are contaminating river water, ground water and soil. Therefore, development of new technologies for economical and efficient removal of the contaminants from the environment are in great need, and a variety of new technologies have been proposed for such applications.
For example, a combustion process has been proposed to decompose chlorinated aliphatic hydrocarbon compounds. Although this process is relatively simple, there is a new concern that decomposition products of chlorinated aliphatic hydrocarbons, i.e., hydrogen chloride and chlorine, would react with other organic compounds during the combustion process to produce more toxic substances, such as polybiphenyl chloride and dioxin. Additionally, the high temperature treatment is a drawback in view of energy consumption.
Also, techniques of using an oxidizing agent or a catalyst for decomposing chlorinated aliphatic hydrocarbon compounds have been proposed. Specific examples include a process of decomposing harmful hydrocarbons using ozone (Japanese Patent Application Laid-Open No. 3-38297), a process of wet oxidation decomposition at high temperature under high pressure and a process of oxidization decomposition with hydrogen peroxide or a salt of iron (Japanese Patent Application Laid-Open No. 60-261590).
There is also proposed a process of using sodium hypochlorite as an oxidizing agent (U.S. Pat. No. 5,611,642), or a combined use of sodium hypochlorite and UV irradiation (U.S. Pat. No. 5,582,741). A process is also proposed in which a suspension of a photocatalyst (fine particles of an oxide semiconductor such as titanium oxide) in liquid chlorinated aliphatic hydrocarbons is prepared under alkaline conditions to decompose the hydrocarbons by light irradiation (Japanese Patent Application Laid-Open No. 7-144137). A catalytic oxidation degradation process using an oxide, such as platinum oxide, alumina and zirconium oxide, is also proposed (H. Ichimura et al., Japanese Patent Application Laid-Open No. 6-31135).
Also, there has been tried a photolysis process in which chlorinated aliphatic hydrocarbon compounds are decomposed by irradiation with UV light in a gaseous phase without using an oxidizing agent (H. Seki et al, xe2x80x9cContaminated Groundwater and Soil: Present Condition and Countermeasurexe2x80x9d, ed. by Kansai Branch of Japan Water Environment Society and Environmental Technology Research Institute, 1995; Japanese Patent Application Laid-Open No. 8-243351).
It is known that chlorinated aliphatic hydrocarbons such as TCE and PCE are decomposed by microorganisms aerobically or anaerobically, and there have been attempts to decompose such compounds and purify the natural environment by such a microbial process.
In Japanese Patent Application Laid-Open Nos. 5-269346 and 5-068845, there are disclosed methods for removing compounds of low boiling points by using adsorbents, such as activated carbon and zeolite. These methods do not mention how to treat the adsorbed contaminants. Japanese Patent Application Laid-Open No. 8-24572 discloses a decomposing method where a contaminant once adsorbed on activated carbon is released by heating it and then decomposed; the methyl bromide-containing gas exhausted from a fumigation warehouse is adsorbed by activated carbon and then methyl bromide released from the activated carbon by heating is subjected to thermal decomposition in the presence of a catalyst, such as alumina, titania, and zirconia. Japanese Patent Application Laid-Open Nos. 5-290, 5-291 and 5-292 disclose methods of degrading organochlorine compounds adsorbed by porous iron material, where hydrogen peroxide is added to a waste water containing organochlorine compounds, such as trichloroethylene, and the waste water is circulated and aerated in a treatment apparatus provided with an iron porous material to carry out adsorption-oxidative decomposition of the compounds.
Further, Japanese Patent Application Laid-Open No. 5-317716 discloses a method for treating waste water with a porous catalyst, where a porous material is prepared by sintering iron oxide with at least one element selected from cobalt, nickel cerium, silver, gold, platinum, palladium, rhodium, lutenium and iridium, and, using the porous material as a catalyst, wet oxidation of nitrogen-containing or sulfur-containing or organohalogen compounds adsorbed by the porous material is carried out.
As listed above, a number of methods for decomposing halogenated aliphatic hydrocarbon compounds have been proposed to date. However, the present inventors have found or predicted problems that most of these methods require complicated apparatuses for decomposition, and/or a further process to make the decomposition products harmless. Thus, there is still a strong demand for eco-friendly technology for decomposing halogenated aliphatic hydrocarbon compounds.
The present invention was made based on the inventors"" new finding, and the object of the present invention is to provide a method of efficiently decomposing halogenated aliphatic hydrocarbon compounds in an eco-friendly manner with a lower possibility of producing decomposition products that will cause secondary environmental contamination, as well as an apparatus to be used for such a method.
In a study for achieving the above objects, the present inventors found that functional water obtainable by electrolysis of water, e.g., acidic water, a sterilizing effect (Japanese Patent Application Laid-Open No. 1-180293) and cleansing effect of which on the surface of semiconductor wafers (Japanese Patent Application Laid-Open No. 7-51675) have been reported, can remarkably accelerate the decomposition of halogenated aliphatic hydrocarbon compounds under light irradiation.
The decomposition speed of halogenated aliphatic hydrocarbon compounds by light-irradiated functional water is very high, but the decomposition efficiency will decrease as the concentration of the compound to be degraded falls, so long as the decomposition reaction is a chemical reaction.
The inventors of the present invention have noted that the time necessary for the reduction of the residual contaminant concentration by one order, for example, from 99% to 99.9% or from 99.9% to 99.99%, would not differ much. They have found that when the decomposition reaction in the decomposition apparatus is stopped at an appropriate stage and the contaminant is adsorbed to reduce the contaminant level in the exhaust gas or waste water to the exhaust standard or lower, and then the adsorbed contaminant is released and returned to the decomposition apparatus, a higher decomposition efficiency can be maintained than when the residual concentration of the contaminant is reduced at a stretch to the exhaust quality standard level or lower, which takes a long time.
Thus, according to an aspect of the present invention, there is provided a method of decomposing a target substance being a halogenated aliphatic hydrocarbon compound with functional water, comprising the steps of:
contacting the target compound and an active component of the functional water for decomposition in a closed reaction chamber wherein the active component is generated in the functional water by light irradiation;
removing the target compound remained intact in the reaction chamber;
adsorbing the removed target by using an adsorbent;
releasing the adsorbed target substance from the adsorbent; and
feeding back the released target substance to the reaction chamber.
According to another aspect of the present invention, there is provided an apparatus for decomposing a halogenated aliphatic hydrocarbon compound comprising:
a reaction chamber in a tightly closed state;
means to supply a target substance being a halogenated aliphatic hydrocarbon compound to the reaction chamber;
means to supply a functional water to the reaction chamber;
means for irradiating light to generate an active component for decomposition in the functional water;
means for removing the target substance remained intact in the reaction chamber from the reaction chamber;
means for adsorbing the removed target substance with an adsorbent;
means for releasing the adsorbed target material from the adsorbent; and
means for feeding back the released target substance to the reaction chamber.
According to the present invention of the process and apparatus for decomposing halogenated aliphatic hydrocarbon compounds, where the target substance for decomposition contained at a low concentration in the exhaust or discharge from the decomposition unit is adsorbed by an adsorption unit, and at regular intervals, the adsorbed target substance is released from the adsorption unit and returned to the decomposition unit, the concentration of the target substance in the waste or discharge can be easily reduced to the environmental standard or lower, avoiding poor running efficiency such as enlarging the size of the decomposition vessel, elongating the residence time, or connecting similar apparatuses in tandem.